There are several types of optical disks and they are classified into three kinds of a play-only optical disk, a write-once optical disk and a rewritable optical disk, in accordance with the recording/reading methods.
Among these, as shown in FIG. 1, the play-only optical disk has a structure in which, after recording data are formed owing to patterned recording pits disposed on a transparent resin base (for instance, polycarbonate) during production, a reflective film layer containing Al, Ag or Au as a main ingredient is disposed. At the time of reading the data, phase difference or reflection difference of laser light irradiated on the disk is detected to reproduce the data. Furthermore, there is another type in which a base where individual recording pits are formed and then a reflective film layer is disposed thereon and a base on which a translucent reflective layer is disposed are adhered and data recorded in the two layers are read. When the recording/reading method is carried out on one surface, data are exclusively used for reading data (incapable of writing and changing), and, examples of optical disks adopting the above-mentioned method include CD-ROMs, DVD-ROMs, BD-ROMs and HD-DVD-ROMs. FIG. 1 is a schematic diagram showing a sectional structure. In FIG. 1, reference numerals 1, 2, 3, 4 and 5, show a polycarbonate base, a translucent reflective film layer (Au, Ag alloy, Si), an adhesive later, a total reflective film layer (Ag alloy) and a UV-curable resin protective layer, respectively.
In such play-only optical disks, disks are mass-produced by means of a press-forming method employing a stamper on which an information pattern is previously formed in the course of forming a disk. Accordingly, it has been difficult to provide an ID number for individual disks. However, in order to inhibit illegal copy of disks, to improve the traceability in a commodity circulation and to add an added value, disks provided with an ID number for individual disks by use of a dedicated unit during production of the disks or after formation of the disks come to be standardized by means of a label gate method or a BCA (Burst Cutting Area) method, in the play-only optical disks as well. At present, ID numbers are marked, mainly by a method in which laser light is irradiated on the produced disks to melt an Al alloy of a reflective film, followed by making a hole (void) in the reflective film to thereby carry out the recording.
As reflective films of the play-only optical disks, so far, Al alloys centering on JIS6061 (Al—Mg base alloy), which is much in a turn volume as a general structural material and accordingly less expensive, has been broadly used. Furthermore, recently, Ag alloys having high reflectance in comparison with that of the Al alloys have been under study. Ag alloy reflective films have been broadly used in recording optical disks such as DVD-Rs and CD-Rs.
However, these materials have a high thermal conductivity. Accordingly, when these materials are used in the play-only optical disks that are marked with laser, a higher laser power is necessary to mark, and therefore, there are problems in that the polycarbonate substrate that is a base and an adhesive are thermally damaged. Furthermore, when Ag alloy is used, since Ag is low in the heat resistance, there is a problem in that Ag film agglutinates at high temperatures to thereby lower the reflectance.
With regard to a decrease in the thermal conductivity of Ag alloy, JP-A-4-252440 discloses a method where Ge, Si, Sn, Pb, Ga, In, Tl, Sb or Bi is added to Ag to thereby lower the thermal conductivity. Further, JP-A-4-28032 discloses a method where Cr, Ti, Si, Ta, Nb, Pt, Ir, Fe, Re, Sb, Zr, Sn or Ni is added to Ag to thereby lower the thermal conductivity. However, the above-mentioned reflective films intend to inhibit heat from diffusing from a recording layer. Accordingly, since a function as the reflective film is lost when the reflective film itself is thermally modified, the reflective film is assumed not to be modified. On the other hand, in order to melt and remove a film by laser irradiation to thereby record a high quality signal, it is necessary that not only the absorptivity of the laser, the melting temperature and the thermal conductivity are optimized to reduce the recording power but also a melt region has to be controlled so that a shape thereof may be clear and a melt residue may not remain therein. However, an Ag alloy thin film satisfying the demand from a viewpoint of a mark shape during laser marking has not been obtained.
Patent document 1: JP-A-4-252440
Patent document 2: JP-A-4-28032